The present invention describes a system for the conversion of nitrous oxide into a breathable gas mixture consisting of nitrogen and oxygen. The system may optionally be fitted with an air separation membrane to enrich the breathable gas in oxygen.
Self-contained breathing apparatus (SCBA, or SCUBA, for the underwater variety) systems are currently used in an enormous number of applications. People such as fire fighters and hazardous (nuclear, biological and chemical) material workers regularly find themselves in unhealthy and even toxic atmospheres, and have to wear air or oxygen tanks. SCBA systems are also an essential part of the spacesuits used by NASA astronauts in extravehicular activities. Many military planes and submersibles carry a supply of liquid oxygen to provide breathing gas for the crews. High altitude climbers regularly use compressed oxygen supply systems. Finally, underwater SCUBA systems are used by various people, including military divers, underwater workers, search and recovery divers, and more than 3 million recreational divers in the United States alone.
Current SCBA systems primarily utilize compressed air. This works, but suffers from excessive mass, as the air tanks must be built to withstand several thousand pounds per square inch (psi) internal pressure. In addition, supply systems are inherently dangerous at these pressures, as tanks or fittings can burst. Finally, the nitrogen and oxygen in air are permanent gases at ambient temperatures, which means that they are very low density even at extremely high pressures ( greater than 200 bar), unless they are cryogenically liquefied. Thus, SCBA systems are either heavily insulated cryogenic tanks or very large and heavy gaseous air tanks.
For example, a common fire fighting air pack includes a compressed (standard atmospheric composition) air tank designed for thirty minutes duration, although, in the real world of excitement and exertion, this is usually somewhat optimistic. The tank contains 1.53 standard m3 of air compressed to 4500 psi (310 bar). Thus, this tank, which weighs about 23 kg, holds about 0.5 kg of oxygen.
Standard SCUBA tanks typically contain 11.1 liters of an oxygen/nitrogen mix (79% nitrogen, 21% oxygen, standard atmospheric mix) under about 3000 psi (207 bar), which is approximately 2.27 standard m3. This provides a total of about 2.9 kilograms of air (0.68 kg of which is oxygen) which is usually enough for about 30 minutes to one hour underwater. Underwater diving is different from use on land because of the increased pressure at diving depths. This has two effects. First, because of the higher density of the gas at higher pressures, each breath, assuming a constant volume, consumes more of the stored air. The second effect is that at higher pressures, nitrogen present in the air mixture is dissolved into the blood. While at diving depth, this is of little concern, but if the diver attempts to return to the surface too quickly, the nitrogen will quickly come out of solution and form bubbles in the bloodstream. This phenomenon is the cause of xe2x80x9cthe bends,xe2x80x9d a familiar diving problem that in extreme cases can cause severe pain and death. The result of both of these effects is that the deeper a diver is, the less time he/she can remain there.
To help alleviate the problems associated with diving at depth, a SCUBA mix called xe2x80x9cnitrox,xe2x80x9d is used. Nitrox is enriched to (depending on intended use) 32% to 36% percent oxygen with the rest nitrogen. The increased oxygen content reduces both the absorption of nitrogen in the blood and the quantity of the gas mixture that must be inhaled with each breath, and thus extends the permissible depth and length of dives. However, it is still limited in that a portable tank cannot hold very much breathable gas.
Nitrous oxide is not by itself a breathable gas. Nitrous oxide has a long history of use as an anesthetic. It was first used for surgery in 1799, but found its primary employment in dental anesthesia, where it was first used in 1844. However, anesthetic applications typically employ N2O in a 60%-75% concentration with air to achieve the desired effects. In concentrations of less than 30%, N2O is incapable of causing deep anesthesia.
Nitrous oxide has recently received increased attention as a greenhouse gas pollutant and as a contributor to ozone destruction. The major manmade source of nitrous oxide emissions is from the production of adipic acid, which is used in the synthesis of nylon monomers. Several methods have been researched for catalytically decomposing N2O from these sources into nitrogen and oxygen to limit the negative environmental effects. Since N2O mole percentage in these streams is generally less than 10%, the majority of research has focused on nitrous oxide decomposition in dilute amounts.
U.S. Pat. No. 5,137,703 claims a method for thermal catalytic decomposition of nitrogen oxides into molecular oxygen and nitrogen using a variety of catalysts, including mixtures of noble metals, transition metal oxides and group III metal oxides. This patent is primarily aimed toward decomposition of NO and N2 waste gases from industrial power plants.
U.S. Pat. No. 5,171,553 describes catalyst activities specifically for the decomposition of N2O. The patent describes the use of common noble metal catalysts on inert supports such as alumina. The supports were replaced with noble metal-exchanged crystalline zeolites. While the zeolite without the metal atoms was non-catalytic toward N2O decomposition, when metal-exchanged it provided superior performance over standard catalyst supports.
U.S. Pat. No. 5,314,673 describes a method for decomposition of streams of up to 100% N2O over a tubular reactor filled with cobalt oxide and nickel oxide on zirconia catalyst.
There is a need for a breathable gas mixture which can be carried in a compact fashion and which can last a long time.
There is a need for a compact source of breathable gas which is safe and easy to use.
There is a need for a portable, breathable gas supply unit which will supply gas for a longer period of time than traditional SCBA tanks.
There exists a need for a portable supply tank of gas which can be oxygen enriched.
There exists a need for supplying warmth to underwater divers or astronauts under certain conditions.
The present invention is a complete system for efficiently decomposing nitrous oxide to nitrogen and oxygen, and supplying the resulting gas mixture to a breathing apparatus. The portable system consists of a compact nitrous oxide supply system, a catalytic decomposition reactor, a breathable gas storage reservoir, a mouthpiece, and appropriate controls. It may optionally include an air separation membrane to enrich the breathing gas in oxygen before supplying it to the user. The system can be used in a variety of locations and for numerous applications, including those listed herein.
High conversion of nitrous oxide to nitrogen and oxygen can be achieved by utilizing an oxidized metal catalyst on an inert support or a metal substituted crystalline zeolite at temperatures above 250xc2x0 C. A preheat to at least 150xc2x0 C. is required to raise the reactor temperature to the desired level, after which energy from the exothermic decomposition of the nitrous oxide is sufficient to maintain the reaction temperature. The preferred preheat configuration uses an internal or external electrical heating element, but chemical ignition or other heating methods are also possible.
The decomposed nitrous oxide can be supplied directly to the breathing system user or it can be optionally enriched using a selective membrane. The oxygen levels from nitrous oxide decomposition can be boosted above 70 mole percentage with a once through air separation membrane that separates oxygen from the nitrogen and residual N2O in the catalytic reactor exhaust. The warm N2O gas produced can also be used to provide heat to a dry suit or spacesuit prior to consumption.